I think the assumption is that Rugby is American Football without the body armour, but in reality they are immensely different sports especially in the way that you can tackle and the pacing of the game. American Football, due to its stop/start nature, plays into faster more aggressive tackling which requires all of the protection. Rugby has a more even flow to the game and as such the pace is often slower and tackles tend to have less impact. In Rugby you're looking to hold someone in place more than anything else, and properly tackling someone to the floor is usually not the best course of action (because whilst your down on the floor, the ball's been passed to someone else who's ran past you).